Hydrogeology and simulated ground-water flow through the unconsolidated aquifers of northeastern St. Joseph County, Indiana

The U.S. Geological Survey investigated ground-water resources of northeastern St. Joseph County, Indiana, during 1990-93. The investigation included field measurements of water levels and numerical models of ground-water flow. This report documents results of that work and includes descriptions of (1) hydrogeologic framework, (2) water levels, (3) model sensitivity to variations in hydrogeologic parameters, (4) simulated aquifer response to increased ground-water withdrawals, (5) recharge areas for significant water- withdrawal facilities, (6) flow paths and discharge points for ground-water solutes originating beneath known contamination sites. Water-level data indicated (1) regional ground- water flow towards the St. Joseph River, (2) depth to water is small in the St. Joseph aquifer system compared to that in the Hilltop and Nappanee aquifer systems, (3) water levels in deep and shallow parts of the aquifer system are not equal where a confining unit is present. Model results indicate increasing withdrawals by 50 percent at significant water-withdrawal facilities would cause drawdowns less than 6 feet in the 1/4-square-mile area surrounding pumping sites. The response of Juday Creek and the St. Joseph River to increased ground-water pumpage is reductions of ground-water contribution to streamflow of 23 percent and 6 percent, respectively. Particle-tracking analyses indicate flow paths for solutes originating beneath known contamination sites may pass near to, or be intercepted by, significant water-withdrawal facilities. Most particles are discharged to the St. Joseph River but some may be discharged to Juday Creek.